1. Field of the Invention
This invention relates to a bearing unit which supports a rotary shaft for rotation or supports a rotary member for rotation on a shaft and a motor which uses a bearing unit.
2. Description of the Related Art
Various bearing units for supporting a rotary shaft for rotation are known, and an exemplary one of bearing units in related art has such a configuration as shown in FIG. 18.
Referring to FIG. 18, the bearing unit 100 shown supports a rotary shaft 101 for rotation and includes a radial bearing 104 for supporting the rotary shaft 101 in a circumferential direction and a housing 105 in which the radial bearing 104 is accommodated.
In the bearing unit 100, the radial bearing 104 cooperates with lubricating oil, which is viscous fluid filled in the housing 105, to form a dynamic pressure fluid bearing. Dynamic pressure generating grooves 111 for generating dynamic pressure are formed on an inner circumferential face of the radial bearing 104 in which the rotary shaft 101 is fitted.
The housing 105 in which the radial bearing 104 on which the rotary shaft 101 is supported is accommodated is a single member which has such a shape that it accommodates and surrounds the radial bearing 104 formed in a cylindrical shape as seen in FIG. 18 and is formed by integral molding of a synthetic resin material.
The housing 105 includes a housing body 106 having a cylindrical shape, a bottom closing portion 107 formed integrally with the housing body 106 so as to close up a first end side portion of the housing body 106 and forming a first end side portion, and a top closing portion 108 formed integrally with the housing body 106 and forming a second end side portion of the housing body 106. The housing 105 formed in this manner is formed as a unitary member such that the radial bearing 104 is disposed in the inner circumference side of the housing body 106 by outsert molding synthetic resin material such that the housing 105 surrounds the radial bearing 104 of a cylindrical shape.
A shaft fitting hole 109 is provided at a central portion of the top closing portion 108, and the rotary shaft 101 supported for rotation on the radial bearing 104 accommodated in the housing 105 is fitted in the shaft fitting hole 109. A thrust bearing 110 is formed integrally at a central portion of the inner face of the bottom closing portion 107, and a bearing supporting portion 102 provided on the first end side in the thrust direction of the rotary shaft 101 supported on the radial bearing 104 is supported for rotation on the thrust bearing 110.
The thrust bearing 110 is formed as a pivot bearing which supports the bearing supporting portion 102 of the rotary shaft 101, which is formed in an arcuate shape or a tapering shape, at a point.
The shaft fitting hole 109 is formed such that it cooperates at a circumferential face thereof with an outer circumferential face of a shaft body portion 103 of the rotary shaft 101 to define therebetween an air gap 112 of a distance sufficient to prevent lubricating oil 113 filled in the housing 105 from leaking from within the housing 105.
A tapered portion 114 is provided on an outer circumferential face of the rotary shaft 101 opposing to the inner circumferential face of the shaft fitting hole 109. The tapered portion 114 provides a pressure gradient to the air gap 112 formed between the outer circumferential face of the rotary shaft 101 and the inner circumferential face of the shaft fitting hole 109 to generate force acting to suck the lubricating oil 113 filled in the housing 105 into the inside of the housing 105 thereby to prevent leakage of the lubricating oil 113 filled in the housing 105.
In the bearing unit 100 configured in such a manner as described above with reference to FIG. 18, the rotary shaft 101 is exposed only at one end thereof adjacent the shaft fitting hole 109 but is covered seamlessly with the housing member except the small gap at the shaft fitting hole 109. Therefore, the bearing unit 100 can prevent leakage of the lubricating oil 113 to the outside of the housing 105. In other words, the bearing unit 100 allows assured prevention of leakage when compared with an alternative arrangement wherein a housing is formed by assembling two or more housing members by adhesion by means of a sealing material such as bonding agent. Further, since only the gap of the shaft fitting hole 109 serves as a communicating portion with the outside, scattering of lubricating oil by impact can be prevented.
However, in the bearing unit 100 described above, when the rotary shaft 101 and the housing 105 rotate relative to each other to generate dynamic pressure, the static pressure in the inside of the bearing unit 100 decreases extremely. As the pressure in the inside of the housing 105 decreases, remaining air such as air remaining by a very small amount in the inside of the housing 105 and air dissolved in viscous fluid such as lubricating oil is expanded. Consequently, the viscous fluid may possibly be extruded to and leak from the exposed portion side of the shaft. In the bearing unit 100, if the lubricating oil leaks and can hardly be retained, then good lubrication may not be maintained.
In this manner, the bearing unit 100 shown in FIG. 18 has a problem in that, when dynamic pressure is generated by relative rotation of the rotary shaft 101 and the housing 105, the remaining air in the inside of the housing 105 may be expanded to cause the lubricating oil to leak.
A bearing unit which solves the problem just described is disclosed, for example, in Japanese Patent Laid-open No. 2005-69382. The bearing unit includes, in addition to the configuration of the bearing unit described above, a communicating path for communicating the first and second end sides in the thrust direction of the rotary shaft projecting from the radial bearing with each other. The communicating path prevents drop of the pressure on the closed end side of the shaft. Further, the communicating path discharges therethrough the remaining air in the housing from the shaft fitting hole thereby to prevent leakage of the lubricating oil.
However, also with the bearing unit described, when the remaining air in the housing is to be discharged to the outside from the bearing insertion hole through the communicating path, as the remaining air advances from the closed end side toward the open end side of the shaft, bubbles of the remaining air sometimes gather together to form bubbles of an increased bubble size. Where such bubbles of an increased bubble size are produced, they may possibly stay within the communication path, resulting in failure in smooth discharge of the bubbles to the outside from the communicating path. The remaining air sometimes gives rise to leakage of the lubricating oil.